FIG. 5 shows a perspective view of a prior art infrared imaging device. In FIG. 5, a two-dimensional array of photodiodes 1 comprising such as 128.times.128 pixels is provided, which detects the infrared light 4 which is incident thereon and converts the light to electrical charges. A charge transfer device 2 is provided for successively transferring charges which are output from the two-dimensional array of photodiodes 1. In this case, this charge transfer device is constituted by a CCD (Charged-Coupled Device) which is produced on a silicon substrate. Indium bump electrodes 3 electrically connect the two-dimensional array of photodiodes 1 and the CCD 2.
FIGS. 6(a) to 6(c) schematically illustrate an operation of a charge input circuit of the CCD 2 of the infrared imaging device shown in FIG. 5. In FIG. 6, the same reference numerals as those shown in FIG. 5 designate the same or corresponding parts. An input gate electrode 5 is provided for controlling the input of the signal charge to the change input circuit of the CCD 2. A storage gate electrode 6 is provided for storing charges input to the charge input circuit. A transfer gate electrode 7 is provided for transferring the charges to the initial stage of the CCD 2. A driving electrode 8 is provided for driving the CCD 2. A skimming gate electrode 9 is provided for supplying a skimming voltage for establishing a skimming level in the potential well below the storage gate electrode 6. Reference numeral 11a designates stored charge remaining in the potential well not exceeding the skimming level. Reference numeral 11b designates stored charge in the potential well which has exceeded the skimming level. A charge exhaustion gate electrode 10 is provided for controlling the transfer of the stored charge 11a to the outside. Reference numeral 25 designates a charge exhaustion electrode.
A description is given of the operation hereinafter.
The two-dimensional array of photodiodes 1 and the CCD 2 are electrically connected by indium bumps 3 and a voltage different from the voltage applied to the storage gate electrode 6 is applied to the skimming gate electrode 9 thereby to produce a potential barrier in the charge storage section.
Here, when the infrared light 4 is incident on the two-dimensional array of photodiodes 1, the charges 11 produced by the photodiodes 1 flow into the input gate electrode 5 of CCD 2 via the indium bump 3 and are stored below the storage gate electrode 6 for a predetermined time (FIG. 6(a)). Thereafter, by changing the voltage applied to the transfer gate electrode 7, the stored charge 11b is transferred to below the CCD driving electrode 8 (FIG. 6(b)). At this time, a predetermined quantity of stored charge 11a which does not exceed a potential barrier remains without being transferred because of the potential barrier produced by the skimming gate electrode 9 at the storage gate electrode 6. Thereafter, by operating the exhaustion gate electrode 10, the stored charge 11a is exhausted to the charge exhaustion electrode 25 (FIG. 6(c)).
FIG. 7 shows a conceptional view in which a construction of the device is shown in a plane. Reference numeral 15 designates the charge input circuit of the CCD 2 shown in FIG. 6(a) .
This method is described by Chow et al. in IEEE Transactions on Electron Devices, Volume ED-29, Number 1, January 1982, pages 13-4, and is called as "a charge skimming technique". This technique is effective when signal contrast is low as in the case of imaging infrared light.
FIG. 8 shows the content of the stored charged 11 in the imaging of infrared light of about 10 microns wavelength. Because the quantity of charge 13 generated by background radiation is much larger than the quantity of charge 12 generated by the signal light, the signal contrast is low. In the above described charge skimming technique, by giving an appropriate voltage to the skimming gate electrode 9, the skimming level 14 is determined as shown in FIGS. 9(a) and 9(b) thereby to remove the charge generated by the background radiation, and the charges 12 generated by the signal light are effectively transferred.
Therefore, an image of high signal contrast and high quality is obtained.
In the infrared imaging element utilizing above described prior art charge skimming technique, the skimming gate voltage has to be the same for all pixels. Furthermore, in a case where the skimming gate voltage is applied to respective pixels independently, the wiring of all of the 128.times.128 pixels has to be taken out to the outside. Such complex wiring is physically impossible.
The prior art solid-state imaging device utilizing the charge skimming transfer technique is constructed as described above and because the skimming gate voltage is the same for all pixels, when the sensitivity of the photodiodes is not uniform throughout all pixels as shown in FIG. 10(a), different pixels produce different charge quantities and the skimming gate voltage 14 has to be set in accordance with the pixel having the lowest sensitivity . As a result, charges produced at respective pixels is as shown in FIG. 10(b) after the skimming and it is impossible to effectively remove the d.c. component due to the background radiation from all pixels and therefore, it is impossible to perform an effective charge skimming transfer.
As a solution to this problem, a method of establishing a separate skimming gate voltage for each respective pixels is considered. However, in order to establish the skimming level separately for each pixels, the wirings of each of the 128.times.128 pixels has to be taken out to the outside. Furthermore, it is necessary to input the established values of skimming levels for each pixel through wiring and it is physically impossible to design that wiring.
A solution to this problem is proposed in Japanese Patent Publication Hei. 2-151183. In this device a skimming CCD for transferring the skimming voltage as a time sequence signal is arranged in parallel with the vertical CCDs for charge skimming transfer, the voltage level in proportion to the charge due to the background radiation of the respective pixel is measured, this is once stored in an external memory, and the stored signal is applied in sequence to the skimming electrodes of the respective pixels by the successive transfer by the CCD for skimming voltage transfer. However, this device requires an external memory for establishing the skimming level, a D/A converter, and an A/D converter, and a CCD for skimming voltage transfer has to be provided in parallel with the vertical CCD, thereby complicating the structure. Furthermore, because the skimming level is once stored in an external memory, it can not correspond to changes in the sensitivity of the pixel which instantaneously, occurs and it is impossible to reflect a change in sensitivity of the pixel on the skimming level in real time.